Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

606
FXUS63 KLOT 211140
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CDT Tue Apr 21 2026

.KEY MESSAGES...

- Threat for scattered thunderstorms this afternoon and evening.
  Severe threat is low, but a few strong/gusty cores possible,
  particularly south of I-80 and east of I-55.

- Warming trend through Thursday (except cooler near Lake
  Michigan on Wednesday).

- More widespread showers and thunderstorms with another cold
  front Thursday night into Friday.

&&

.DISCUSSION...
Issued at 218 AM CDT Tue Apr 21 2026

Water vapor loops reveal a low-amplitude shortwave embedded
within broad northwest flow, shifting steadily eastward into
northwest Iowa and southwest Minnesota early this morning. Ahead
of this feature, a southwesterly LLJ is near its nocturnal
peak, but will remain robust as it drifts across parts of the
region later this morning. Expectation is for surface gusts to
increase quickly after sunrise as mixing into the base of this
arriving speed max begins. Based on the progression and
orientation of the strongest flow, there will likely be a
notable north- south wind gust gradient across the area today,
with the strongest occurring roughly east of I-55 and south of
I-80. Given fairly deep mixing (at least to 850-800 mb), have
boosted wind gusts towards the NBM 90th percentile (35+ mph),
which aligns better with mean boundary layer flow near 30 knots
during the afternoon. In addition, have dropped dewpoints across
the southeast third of the forecast area, in-line with the
deeper-mixing guidance as dewpoints mixed out pretty
aggressively yesterday. With the overlap of stronger winds and
afternoon RH values near 30 percent, parts of the area may
approach an elevated grass fire danger as fine fuels have dried
(Midewin RAWS site at 7% yesterday afternoon), and this part of
the forecast area hasn`t received nearly as much rain as farther
north. Will highlight this in the fire weather planning
forecast.

The other focus today is on what appears to be an increasing
potential for thunderstorms this afternoon and into the evening.
The aforementioned shortwave will transport a renewed elevated
mixed layer into the area later this morning and afternoon. The
upper air sounding at North Platte, NE sampled the core of this
EML plume as it advected off the high terrain, with surface-5 km
lapse rates near dry adiabatic. As this plume of steep lapse
rates arrives in our northwest through the morning, forecast
guidance nearly ubiquitously shows increasing saturation taking
place at the top of the EML near 600-500 mb along with the
eventual initiation of very high-based convection rooted above
6-7 kft. Convection initially may remain fairly sparse, but it`s
now looking like at least widely scattered coverage is in the
cards across parts of northeast Illinois and into northwest
Indiana during the afternoon hours.

As this activity continues southeastward, there may be a
potential for some stronger wind gusts as cores work into an
area of deeper mixing. Overall, instability will not be that
significant (likely a few hundred J/kg), but the dry sub-cloud
layer may promote some strong gusts to the surface, particularly
south of I-80 and east of about I-55. If cores manage to
sustain and grow (which also is not entirely clear at this
point), couldn`t rule out the potential for a few 60+ mph gusts
and perhaps some small hail. Additional convection may develop
this evening as a southwesterly LLJ impinges upon some increased
925-850 mb moisture and overtops a remnant outflow boundary.
The orientation of the mid and upper level wind field (strong
veering to the northwest with near-surface southerlies) can
result in localized training issues. So while the multi-model
CAM suite really doesn`t show a strong signal for this now, will
need to keep an eye on this in the vicinity of the IN/IL state
line this evening.

A backdoor front/boundary will push into the region through late
tonight. This will turn winds onshore at the lakefront tonight.
Can`t rule out that areas of fog bleed in off the lake late
tonight into Wednesday morning, but neutral to mixy modified
Richardson number values increase uncertainty as to how far
inland this threat will extend. For now, have confined fog
wording to the first mile or two of the lakefront.

Onshore flow will persist through the day on Wednesday, and the
combination of stratus and fog over the lake will likely hold
high temperatures in the mid 50s lakeside while inland locales
will warm into the 70s. This boundary looks like it`ll stall
somewhere in our southwest through the day, and with dewpoints
forecast to rise to near 60 to its south, additional
thunderstorm development will be possible through midday and
afternoon hours Wednesday. Forcing appears very nebulous, and
likely limited to general near-surface convergence, so made no
alterations to the NBM-delivered slight chances in our far
southeast at this time.

Warmer and breezy conditions will develop on Thursday, with
lingering lake cooling likely confined to portions of Lake
County (IL) and perhaps far northeastern Cook. The next
synoptic trough will roll overhead through Thursday night
delivering another round of more widespread showers and some
thunderstorms into Friday morning. Limited instability, the
climatologically unfavorable time of night, and overall weak
flow suggests the severe threat should remain pretty low with
this activity.

A cold front will send temperatures back to near-normal levels
over the weekend. Another system will bring the next chances
for showers and storms back to the area early next week.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 640 AM CDT Tue Apr 21 2026

Key messages for the 12Z TAF period:

* A few hour period of showers and thunderstorms Tuesday
  afternoon in Chicagoland. Associated gustier winds and vsby
  reductions possible.

* A signal for dense fog/low stratus late Tuesday night into
  early Wednesday. Low confidence on exact impacts.

SSW winds are building early this morning and will see regular
gusts between 20 and 25 kt for most of the daytime as they
gradually veer to westerly.

A broken line of showers and possible thunderstorms is
anticipated to develop over northern IL and drop south across
the Chicago metro this afternoon. Periods of moderate rainfall
with MVFR vsbys will be possible. Localized gusty winds to near
or over 30 kt are also on the table. Favored window for ORD/DPA
is 19-22Z, and 20-23Z at MDW/GYY. While not out of the question,
precip looks to remain east of RFD this afternoon.

Winds will ease and veer to NW behind the showers, then onto NE
by late Tuesday night, likely with some erratic behavior at
times during the evening. Dense fog is favored to develop over
southern Lake Michigan and protrude inland late Tuesday night
into early Wednesday. Noteworthy uncertainty exists in exact
impacts and how late into Wednesday those will linger. At least
a period of MVFR cigs and vsbys is highly favored, although a
strong signal exists for IFR to LIFR conditions after around
09Z, mainly around Chicago and Gary. With the uncertainties,
maintained prevailing low MVFR for the late overnight through
morning. A TEMPO for IFR was introduced at ORD, MDW, and GYY
during this TAF cycle which corresponds to the most likely
window of greatest impacts. Timing on a return to VFR is equally
uncertain. Maintained MVFR through the morning period, but a
signal exists for VFR by mid-morning.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 PM CDT this afternoon for
     Wilmette Harbor to Calumet Harbor IL.

     Small Craft Advisory until 7 PM CDT this evening for the IN
     nearshore waters.

&&

$$

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